Added lib.get_unix_time().

Returns the current time since the Unix epoch as a high-precision
double.
Added app.now() API function to return a time source for timers.
Added CSV app: dump link/app stats to .csv files.
design/csv/csv: Added script to generate CSV files.

Uses an example app network including 10G NICs on chur.
designs/csv: Added sample app.csv and link.csv.
Merge pull request #155 from lukego/csv

Generate CSV files for diagnostics [DRAFT]